Flexible shafts are used for mechanical rotary power drive systems serving a wide range of products, like garden tools, aerospace activation systems, construction equipment, medical devices, and automotive applications. Flexible shafts are commonly used in gasoline powered weed trimmers. The rotating flexible shaft operates inside the outer down-tube, which is an external steel or aluminum outer tube placed between the gasoline engine or electric motor and the cutting head comprising the nylon string cutting line. A casing assembly, more commonly known within the weed trimmer industry as a “liner,” is located between the rotating flexible shaft and this external down-tube. The liner serves several purposes.
This extruded liner conforms to the inside diameter of the down-tube, through both the straight and curved or bend section(s) of the outer down-tube. The liner supports and centers the flexible shaft within the down-tube allowing proper axial alignment of the flexible shaft with respect to the mating engine and cutting head, and the liner acts as a bearing or wear surface for the rotating flexible shaft.
The liner usually comprises a metallic and/or a plastic composition. Over the past decade, the industry has switched almost universally to a star-shaped, all-plastic design due to its lower cost and reduced weight. This current plastic design is produced two ways: as a single extrusion or as a co-extrusion. The single extruded design presently uses a high temperature 6/6 nylon material. The co-extruded design presently uses an inner tube made from high temperature 6/6 nylon material over which a polypropylene star shape is extruded.
The conventional star-shaped liner configuration creates a number of manufacturing and performance problems for weed trimmer manufacturers. The steel outer down-tube, as well as the nylon liner, is produced within standard manufacturing tolerances, wherein the tolerance extremes of the two components either provide an excessively tight fit or an excessively loose interference fit with respect to each other. In the tight fit condition, the star-shaped straight leg profile of the liner has its legs radially emanating from the center, not allowing for sufficient leg flexure. This excessively tight interference fit between the two components can: a) prevent insertion altogether of the liner into the down-tube or b) require a forced insertion where the radially placed legs buckle non-uniformly during insertion. This buckle effect pushes the axis of the liner and flexible shaft off-center, creating a parallel misalignment condition between the flexible shaft and it's mating end components. This results in excessive wear, heat, and premature failure of the rotating flexible shaft under dynamic operating conditions.
On the other hand, excessive clearance, allowing a loose fit, creates a problem in which the liner will slip or shift axially with respect to the outer down-tube during operation, allowing disengagement of the flexible shaft. This undesirable condition requires additional and costly manufacturing steps in order to permanently retain the loose liner within the outer down-tube assembly.
In addition, it is critical that a flexible shaft be adequately lubricated to achieve its anticipated life expectancy. During assembly, the flexible shaft's entire length is uniformly lubricated with grease prior to insertion into the nylon inner tube of the liner. During operation, the helically wound wires of the rotating flexible shaft create a screw effect which will skive grease from the conventional, smooth, cylindrical wall of the liner's inner tube, resulting in the auguring of the grease toward the lower end of the flexible shaft assembly. This allows the upper end of the flexible shaft to lose grease. Eventually, this dry condition will cause failure of the flexible shaft.
The present invention is an improvement over the “state of the art” for both single extruded and co-extruded star liners and other similar liners using plastic or metallic materials.
This invention features a flexible spiral liner of plastic. Although specifically designed for a weed trimmer, it can also be used in other related applications, including but not limited to hand-held snow blowers, power brooms, lawn edgers, hedgers, pruners, chain saws, blowers/vacuums and automotive applications. The spiral liner design of this invention is structurally different from the common star-shaped liner, as illustrated in Table I, below.
TABLE IConventional StarInventive SpiralElementsShaped LinerLinerCross sectionalStraightEllipticalshape of legCross sectional legRadial from CenterTangent to InnerpositionAxisTube ODAxial placement ofLongitudinalHelicallegsCross sectionalCylindricalNon-cylindricalshape of inner tube(Triangular-like,or other shape)
The dimensional size of the inside diameter, outside diameter, wall thickness, and number of legs of the liner in this invention is dependent on application requirements.
The advantages of the spiral liner in this invention are that its flexible legs self-adjust, wherein the liner's elliptical legs will readily compress and conform to the inside diameter of the down-tube despite internal tolerance variations of the two components; its inherent flexibility provides pressure for axial retention of the liner within the outer down-tube, thus eliminating the need for a secondary means of retention; its unique spiral design and flexible legs uniformly center and support the flexible drive shaft on its intended centerline, preventing parallel misalignment with mating end components and premature failure of the flexible shaft during operation; its unique spiral design conforms to the internal diameter within the formed radius bend of the down-tube, thus preventing distortion of the liner; it is easier to insert into the down-tube; its non-cylindrical tube I.D. maintains a uniform distribution of grease along the entire length of the flexible shaft due to grease retention in the helical pockets of the liner; and its improved vibration isolation due to the inherent spring effect of the liner legs.
The foregoing advantages are accomplished by the novel structure of the elliptical shape of the liner legs, vis-a-vis a straight liner leg, which allows flexure in the down-tube providing a desirable interference fit; the leg placement, emanating tangentially from the outside diameter of the nylon tube, vis-a-vis the legs radiating from the centerline of the star-shaped liner, provides a second flexure point. This has a desirable spring effect, both effective and uniform under all tolerance stack-up conditions.
The liner assembly features a triangular-like cross-sectional inner tube disposed within the elliptically shaped legs of the liner. Three longitudinal helical corners, or pockets, are created within this assembly. These pockets retain grease along the entire length of the inner tube, as aforementioned. The grease skived from the liner wall due to the auger effect of the rotating flexible shaft is uniformly collected in these longitudinal pockets located along its entire length. This retards the propagation of the grease to the lower end, allowing the grease to be more uniformly distributed, and maintaining lubrication along the full length of the flexible shaft for the duration of its expected life. During operation, friction of the rotating flexible shaft elevates the temperature of the grease. The grease will soften, thus allowing it to be dispensed more uniformly from the three longitudinal pockets to the flexible shaft. This design eliminates the need for re-lubrication of the flexible shaft during its life expectancy.